fix: allow nested <dt>/<dd> elements if they are within a <dl> element

.changeset/mean-parents-film.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow nested `<dt>`/`<dd>` elements if they are within a `<dl>` element

packages/svelte/src/compiler/phases/2-analyze/visitors/RegularElement.js

@@ -98,6 +98,7 @@ export function RegularElement(node, context) {
 	if (context.state.parent_element) {
 		let past_parent = false;
 		let only_warn = false;
+		const ancestors = [context.state.parent_element];
 
 		for (let i = context.path.length - 1; i >= 0; i--) {
 			const ancestor = context.path[i];
@@ -129,7 +130,9 @@ export function RegularElement(node, context) {
 					past_parent = true;
 				}
 			} else if (ancestor.type === 'RegularElement') {
-				if (!is_tag_valid_with_ancestor(node.name, ancestor.name)) {
+				ancestors.push(ancestor.name);
+
+				if (!is_tag_valid_with_ancestor(node.name, ancestors)) {
 					if (only_warn) {
 						w.node_invalid_placement_ssr(node, `\`<${node.name}>\``, ancestor.name);
 					} else {

packages/svelte/src/html-tree-validation.js

@@ -2,13 +2,14 @@
  * Map of elements that have certain elements that are not allowed inside them, in the sense that they will auto-close the parent/ancestor element.
  * Theoretically one could take advantage of it but most of the time it will just result in confusing behavior and break when SSR'd.
  * There are more elements that are invalid inside other elements, but they're not auto-closed and so don't break SSR and are therefore not listed here.
- * @type {Record<string, { direct: string[]} | { descendant: string[] }>}
+ * @type {Record<string, { direct: string[]} | { descendant: string[]; reset_by?: string[] }>}
  */
 const autoclosing_children = {
 	// based on http://developers.whatwg.org/syntax.html#syntax-tag-omission
 	li: { direct: ['li'] },
-	dt: { descendant: ['dt', 'dd'] },
-	dd: { descendant: ['dt', 'dd'] },
+	// https://developer.mozilla.org/en-US/docs/Web/HTML/Element/dt#technical_summary
+	dt: { descendant: ['dt', 'dd'], reset_by: ['dl'] },
+	dd: { descendant: ['dt', 'dd'], reset_by: ['dl'] },
 	p: {
 		descendant: [
 			'address',
@@ -75,7 +76,7 @@ export function closing_tag_omitted(current, next) {
 /**
  * Map of elements that have certain elements that are not allowed inside them, in the sense that the browser will somehow repair the HTML.
  * There are more elements that are invalid inside other elements, but they're not repaired and so don't break SSR and are therefore not listed here.
- * @type {Record<string, { direct: string[]} | { descendant: string[]; only?: string[] } | { only: string[] }>}
+ * @type {Record<string, { direct: string[]} | { descendant: string[]; reset_by?: string[]; only?: string[] } | { only: string[] }>}
  */
 const disallowed_children = {
 	...autoclosing_children,
@@ -137,12 +138,24 @@ const disallowed_children = {
  * Returns false if the tag is not allowed inside the ancestor tag (which is grandparent and above) such that it will result
  * in the browser repairing the HTML, which will likely result in an error during hydration.
  * @param {string} tag
- * @param {string} ancestor Must not be the parent, but higher up the tree
+ * @param {string[]} ancestors All nodes starting with the parent, up until the ancestor, which means two entries minimum
  * @returns {boolean}
  */
-export function is_tag_valid_with_ancestor(tag, ancestor) {
-	const disallowed = disallowed_children[ancestor];
-	return !disallowed || ('descendant' in disallowed ? !disallowed.descendant.includes(tag) : true);
+export function is_tag_valid_with_ancestor(tag, ancestors) {
+	const target = ancestors[ancestors.length - 1];
+	const disallowed = disallowed_children[target];
+	if (!disallowed) return true;
+
+	if ('reset_by' in disallowed && disallowed.reset_by) {
+		for (let i = ancestors.length - 2; i >= 0; i--) {
+			// A reset means that forbidden descendants are allowed again
+			if (disallowed.reset_by.includes(ancestors[i])) {
+				return true;
+			}
+		}
+	}
+
+	return 'descendant' in disallowed ? !disallowed.descendant.includes(tag) : true;
 }
 
 /**

packages/svelte/src/internal/server/dev.js

@@ -59,17 +59,22 @@ function print_error(payload, parent, child) {
 export function push_element(payload, tag, line, column) {
 	var filename = /** @type {Component} */ (current_component).function[FILENAME];
 	var child = { tag, parent, filename, line, column };
-	var ancestor = parent?.parent;
 
-	if (parent !== null && !is_tag_valid_with_parent(tag, parent.tag)) {
-		print_error(payload, parent, child);
-	}
+	if (parent !== null) {
+		var ancestor = parent.parent;
+		var ancestors = [parent.tag];
+
+		if (!is_tag_valid_with_parent(tag, parent.tag)) {
+			print_error(payload, parent, child);
+		}
 
-	while (ancestor != null) {
-		if (!is_tag_valid_with_ancestor(tag, ancestor.tag)) {
-			print_error(payload, ancestor, child);
+		while (ancestor != null) {
+			ancestors.push(ancestor.tag);
+			if (!is_tag_valid_with_ancestor(tag, ancestors)) {
+				print_error(payload, ancestor, child);
+			}
+			ancestor = ancestor.parent;
 		}
-		ancestor = ancestor.parent;
 	}
 
 	parent = child;

packages/svelte/tests/validator/samples/invalid-node-placement-6/errors.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "node_invalid_placement",
+		"message": "`<dt>` is invalid inside `<dd>`",
+		"start": {
+			"line": 11,
+			"column": 3
+		},
+		"end": {
+			"line": 11,
+			"column": 19
+		}
+	}
+]

packages/svelte/tests/validator/samples/invalid-node-placement-6/input.svelte

@@ -0,0 +1,14 @@
+<dl>
+	<dt>valid</dt>
+	<dd>
+		<!-- dl resets the validation -->
+		<dl>
+			<dt>valid</dt>
+			<dd>valid</dd>
+		</dl>
+		<!-- other tags don't -->
+		<div>
+			<dt>invalid</dt>
+		</div>
+	</dd>
+</dl>